So I'm planning to cosplay Ken Izumi from Chargeman Ken. Already ordered a yellow spandex bodysuit and wondering what would be the best way to sew (or otherwise attach) the K to the suit, making it stretch properly with the fabric... seems like something I might have to be a bit careful about. Any specific method to use, anything I have to be careful about ect. (...Would sewing even be the best way to go about it, or could painting also be an option?) I'm assuming I should use the same type of material for it, right?

While I generally lean toward sewing most of the time, in this case I'd say paint unless you already have experience sewing spandex-y stuff.

Why?
1. the letter is broad, covering a lot of the torso, giving you lots of places to go awry.
2. you ordered a suit. This means trying to sew on a finished garment instead of the freedom of working on one piece or layer without being encumbered by the rest of the garment. Plus a lot of morph suits are really thin spandex, making bad stitches potential holes.
3. the letter itself is very narrow, meaning skinny placement and tight sewing corners and turns.

I suggest making a cardboard template of your torso to stuff into the suit in order to get good stretch while painting. (it also keeps the paint from bleeding through to the back). Paint the WHITE parts first. They probably will need two coats as most white fabric paints are not as opaque as most like. Then do the black, which will cover any wonky edges of your white paint and give you nice crisp lines.

Well, I haven't sewn anything spandex or similar extremely stretchy material before... though now I'm worried about the stuff mentioned. Took a look at both and now I'm kinda stuck between the options (especially because I don't want to have to buy a new suit if I mess it up)

If I do go for paint, anything I should watch out for? Anything that should be done to the paint afterwards to make it last longer? Might buy some similarly-colored spandex/stretchy fabric and fabric paint just to test

Buy FABRIC paint and it won't be an issue.
Don't rely on an applicator tip to 'draw' if you buy a brand that comes that way. Air pockets inside the tube create messy splashes when they happen.
Painters tape can be used on fabric. It's a great tool, use it.
Paint along the tape and from tape toward your area, but never area toward the tape. On fabric tape doesn't seal as well, thus brushing paint towards it can give you a less crisp edge.

Yes practicing on not-your-project is ALWAYS a good plan. In this case an old tee shirt will suffice.
These are things we can't really tell you that you will learn from practice:
- how much or little the paint soaks in or sits on top of the fabric.
- how steady your hand is.
Is your fabric/ garment stretched enough or is the fabric shifting around under your brush?
- is the place you are working sufficient for this project?
- are there human or environmental factors that will come into play?

Yeah, got some fabric paint today along with yellow, black and white fabric, stretching it around some cardboard to test painting it stretched

It worked ok but one issue I noticed was some of the paint would stick to the cardboard underneath after drying, and when removing it it just looks kinda...weird? Maybe it's not gonna be an issue when it's worn and stretched but still a bit unsure... Think I'm gonna try some sewing too (because I'm gonna need a lot of practice for this)